1. Field of the Invention
This invention relates to a pressurized fluidized bed combustion (PFBC) boiler system which attracts expectation as a means for a new coal-fired generation plant of next generation, since SO.sub.x or NO.sub.x contained in an emitted flue gas is remarkably reduced, and the system is provided with a high efficiency.
2. Discussion of Background
In a coal-fired power generation plant employing a PFBC boiler system, lime stone or dolomite is charged into a fluidized bed, and the combustion is carried out at a comparatively low temperature. Therefore, SO.sub.x is fixed as a sulfur compound and is removed, the emission of NO.sub.x is reduced. The plant is provided with a high efficiency, since the power generation is performed by both a steam turbine which is driven by steam produced by the boiler, and a gas turbine which is driven by a filtered pressurized hot flue gas. Accordingly, much effort has been paid in advanced countries for the development of a power generation plant in use of this PFBC boiler system wherein the realization thereof is intended.
The reason why a filtration apparatus for removing dusts of a hot flue gas is necessary for the power generation plant having the PFBC boiler system, is that blades or vanes of a gas turbine should not be eroded by dusts such as ash of coal, when the gas turbine is driven by a pressurized hot flue gas. FIG. 9 is a schematic diagram showing an example of a power generation plant having a conventional PFBC boiler system.
In FIG. 9, a reference numeral 90 designates a PFBC boiler, 100, a pressure vessel which accommodates a main body of a boiler 101, cyclone separators 102 and 103, and a bed material container 104, 110, a flue gas pipe, 120, a gas turbine, 130, a waste heat boiler, 140, a bag filter and 150, a chimney.
The reason why the cyclone separators are mainly adopted in a fluid catalytic cracking apparatus (FCC) that is currently in practice, or a power generation plant of a conventional or under development PFBC boiler system, is that a practical filtration means which can remove dusts of a hot gas, up to a level wherein the blades or vanes of a gas turbine do not erode in a short time, has not been found other than the cyclone separators. The cyclone separator is provided with an advantage wherein the structure thereof is simple, the high temperature design is easy and the dust removing efficiency is high.
In the conventional PFBC power generation plank, for instance, a pressurized flue gas which has been filtered only by the cyclone separators, is led to a gas turbine having an improved abrasion resistance, and the flue gas exhausted from the gas turbine of which temperature is lowered, is filtered by bag filters or by an electrostatic precipitator to satisfy the environmental criterion.
However, when the boiler is in a transient state, a large amount of dusts containing unburnt ingredients are often emitted. At this moment, a phenomenon is apt to cause wherein a dust discharge port of a cyclone separator is clogged (bridged), a large amount of dusts overflow the cyclone separator, and a flue gas containing a high concentration (10 to 30 times as much as design value) of dusts flows into the gas turbine, whereby there is a high danger of eroding the blades or vanes of the gas turbine in a short time.
Accordingly, a construction is intended as an improved power generation plant, as shown in FIG. 10, wherein the erosion of the blades or vanes of the gas turbine is avoided by disposing the cyclone separators as the primary stage filtration, and providing a filtration apparatus with ceramics filters which is excellent in the dust removing efficiency, on the downstream side of the cyclone separators. In FIG. 10, a reference numeral 1 designates a filtration apparatus with ceramics filters. In this plant, the bag filter 140 in FIG. 9 is not necessary, since the dust removing efficiency of the filtration apparatus with ceramics filters is excellent.
As a large-scaled filtration apparatus with ceramics filters which can be used in a power generation plant, there is a so-called candle type filtration apparatus, proposed in U.S. Pat. Nos. 4,904,287, 5,094,673 and the like, wherein filter tubes are provided of which one-side ends are closed and of which other-side ends are open. Further, tubular type filtration apparatus are proposed of which outline diagram is shown in FIG. 10, and which is provided with a plurality of filter tubes of which both ends are open, and flow passages of a dust containing gas are provided on the inner side of the filter tubes, in Japanese Examined Patent Publication No. 40567/1988, Japanese Examined Patent publication No. 22689/1990, Japanese Examined Patent publication No. 22690/1990, Japanese Examined Patent publication No. 24251/1991, Japanese Examined Patent publication No. 56086/1991, Japanese Examined Patent publication No. 50324/1993, Japanese Unexamined Patent Publication No. 279821/1987, Japanese Unexamined Patent Publication No. 229116/1988, Japanese Unexamined Patent Publication No. 326916/1992 and the like. Further, crossed-flow type filtration apparatus are proposed wherein crossed-flow type filter units are provided, in Japanese Unexamined Patent Publication No. 198606/1990 and U.S. Pat. No. 5,078,760.
In adopting the candle type filtration apparatus as a filtration apparatus with ceramics filters, it is difficult to always maintain a flow of a dust containing gas in the whole region of a filtering chamber wherein the filter tubes are disposed, since a flow area of the dust containing gas in the filtering Chamber wherein the filter tubes are accommodated, is relatively large in this type of filtration apparatus, and there is a tendency wherein a large amount of dusts build up on surfaces of the filter tubes at portions wherein the flow of the dust containing gas is stagnant. Accordingly, it is necessary to remove a considerably large portion of dusts at the primary stage filtration apparatus. A construction is selected wherein cyclone separators having a high dust removing efficiency are employed in the primary stage.
The PFBC boiler system having the filtration apparatus with ceramics filters on the downstream side of the cyclone separators, is devoid of reliability when a transient state is caused. That is, when a large amount of dusts containing unburnt ingredients reach the cyclone separators, the dusts to be removed in the cyclone separatos are apt to temporarily clog dust discharge ports and overflow the cyclone separators. In this case, a flue gas containing high-concentration dusts which have not been removed, flows into the filtration apparatus with ceramics filters, as it is.
At this moment, there often causes a trouble wherein the large amount of dusts containing unburnt ingredients are accumulated on the surfaces of the ceramics filters, especially on the surfaces of the ceramics filters at a portion wherein a gas velocity is retarded whereby the dusts are apt to build up, and the dusts containing unburnt ingredients are ignited and combusted on the ceramics filters. When this combustion is rapidly caused and the ceramics filters are rapidly heated, which exceeds a limit of a thermal shock allowable for the ceramics filters, cracks are generated in the ceramics filters. Further, when the combustion is more violent, the ceramics filter may be damaged by melting.
Such a transient state is caused, for instance, in the operation of rapidly increasing the combustion load, when the gas temperature in the PFBC boiler is 600.degree. C. or less. In spite of a high oxygen concentration, a large amount of unburnt ingredients including soot (in some case as high as 30 wt % of dusts) are emitted, and sometimes, carbon monoxide of which concentration is over 1000 ppm is detected.
Further, in case of the operation wherein the fuel of the boiler is switched from oil to coal, at the start-up of the operation of the boiler, even when the temperature inside the boiler is elevated to about 800.degree. C., a post bed combustion may be caused in the cyclone separators depending on the moisture content of a coal-water mixture or a kind of coal. In this case, the temperature of the dust containing gas being introduced to the filtration apparatus with ceramics filters is suddenly elevated in an amount to 200 through 500.degree. C., which will cause thermal damages of the ceramics filters.
Further, when a large amount of air is blown into the boiler, in an emergency shut down or for the operation to retain perforations of a combustion air distribution plate, the unburnt ingredients in the boiler are blown from the boiler in a large amount, as they are, which overflow the cyclone separators, and the unburnt ingredients reach the filtration apparatus with ceramics filters while they are burning.
Further, the inside of the cyclone separator is provided with a lower narrow portion wherein the dust containing gas swirls and a reaction between the unburnt component and the oxygen in the flue gas is accelerated at that portion. Therefore, the ignition and combustion are also caused in the cyclone separators, the temperature of the flue gas is rapidly elevated and the portion of the cyclone separator may be damaged by melting. Further, when the temperature-elevated flue gas flows into the filtration apparatus with ceramics filters, the ceramics filters may suffer thermal damages.
When the ceramics filters are thermally damaged, the broken pieces of the ceramics filters directly hit and damage the blades or vanes of the gas turbine, the blades or vanes of the gas turbine are eroded by the entrained dusts, and are consumed in a short time period. Therefore, all of these troubles are serious to a power generation plant. These transient phenomena are mostly temporary and caused in a closed room. Therefore, the actual situation thereof is difficult to determine. Conventionally, the breakage of the ceramics filters have been ascribed to its brittleness. However, the inventors have confirmed the presence of these phenomena and the breakage of the filter tubes due to the thermal damage, by a simulated calculation.
In order for the filtration apparatus with ceramics filters to cope with the variable hot dust containing gas, it is necessary to increase the treating capacity of the filtration apparatus with ceramics filters, to provide a means for preventing the rapid elevation of temperature, to frequently perform the regeneration by back-washing, and to find out a means for preventing the accumulation of the dusts containing unburnt ingredients in a large amount. That is to say, it is necessary to provide a filtration apparatus having flexibility whereby the operation is possible without the cyclone separators. However, the necessary expenditure therefor is never small.
However, another big problem is that the necessary cost for installing the PFBC boiler system is enhanced, since plural stages of cyclone separators are accommodated in a pressure vessel which is shared by the PFBC boiler, to achieve the high dust removing efficiency, and therefore, the inner volume of the pressure vessel is increased by the portion of accommodating the cyclone separators. Even if the cyclone separators which treat the hot pressurized dust containing gas are separated from the pressure vessel, it is necessary to make the cyclone separators per se in an expensive bulky pressure vessel, the heat loss dissipated from the surface of the cyclone separator increases, and therefore, the problem can not be solved.
In the filtration apparatus with ceramics filters, to prevent dusts containing a large amount of unburnt ingredients from accumulating in a large amount on the ceramics filters, a mean is effective wherein the filtration apparatus is constructed so that a portion of the dust containing gas is extracted from a dust collecting hopper in the filtration apparatus, the dust containing gas in the filtering chamber of the filtration apparatus flows downwardly, and the layer of the dusts separated from the ceramics filters by the back-washing are swiftly transferred to the hopper. The filtration apparatus with ceramics filters having such a construction, is proposed, for instance, in Japanese Examined Patent Publication No. 24251/1991.
In the tubular type filtration apparatus proposed in Japanese Examined Patent Publication No. 24251/1991, the flow area of the dust containing gas in the filtering chamber is relatively small, and the flow of the dust containing gas is basically a down flow. The dust containing gas is filtered while flowing down in the filter tubes 3, and therefore, the flow velocity of the dust containing gas is almost nullified at around the bottom ends of the filter tubes, when the dust containing gas is not extracted from the hopper.
Moreover, there causes a significant difference in the flow velocity of the dust containing gas among the filter tubes, since it is inevitably influenced by the pressure distribution (there is a difference of approximately 70 mmWC at maximum) and the velocity distribution of the dust containing gas in a gas inlet chamber which fluctuate from moment to moment in accordance with the fluctuation at the upstream portion of the plant. Therefore, the velocity of the downward flow of the dust containing gas may be an up flow at the lower ends of some filter tubes.
In the tubular type filtration apparatus, when the blow down is performed whereby a comparatively small amount of dust containing gas is extracted from the hopper, the downward flow velocity of the dust containing gas in the vicinity of the bottom ends of the filter tubes can be maintained, since the flow area of the dust containing gas in the filtering chamber is relatively small. Accordingly, the quantity of dusts accumulated on the inner surfaces of the filter tubes in the vicinity of the bottom end of the filter can be reduced, the frequency of the regeneration by back-washing of the ceramics filters can be reduced, and the filtration apparatus can maintain a high filtering capacity.
When the quantity of dusts accumulated on the surfaces of the filter tubes is small, normally, a combustion heat enough to thermally damage the filter tubes is not generated, even when the unburnt ingredients are contained in the dusts by approximately 10% and are combusted continually. However, even in the tubular type filtration apparatus, when a large amount of unburnt ingredients which are transiently emitted are introduced, the dusts are accumulated in a large amount on the inner surfaces of the filter tubes in the vicinity of the bottom ends of the filter tubes wherein the downward flow velocity of the dust containing gas is minimized, which are ignited and combusted and thermally damage the filter tubes.
In case of blowing down the dust containing gas from the hopper, it is preferable to return the extracted dust containing gas to the upstream side of the filtration apparatus with ceramics filters, as is proposed in Japanese Examined Patent Publication No. 24251/1991. However, it is difficult to procure a blower which can return the pressurized hot dust containing gas, and a considerable expenditure is required even when it is procured.
The inventors proposed a filtration apparatus wherein a filtration apparatus with ceramics filters is integrated with a compact louver separator for the primary stage dust separation, with the purpose of filtering an emission gas from a blast furnace, previously in Japanese Unexamined Patent Publication No. 279821/1987.
In the louver separator, the dust containing gas is introduced into a space surrounded by a plurality of arranged vanes, the flow direction of the dust containing gas is forcibly changed by the vanes, the dusts having a large particle size or dusts having a large specific weight are moved by inertia in the flow direction of the introduced dust containing gas, or are impinged on the vanes thereby exhausting the kinetic energy and separating the dusts by the gravity. Therefore, the dust removing efficiency is not so high (the dust removing efficiency is 10 to 20% in case of fly ash having a mean particle size of 20 .mu.m).